Such treatments are known to a person skilled in the art and are used in the design of mechanical elements, for example when parts have to rub against one another under severe load and pressure conditions. These treatments can be applied both in cases where the ferrous alloy parts are intended to be lubricated (with oil, with grease, and the like) and in cases where the parts are not intended to be lubricated.
Mention may be made, among the various known treatment processes, of processes for surface oxidation in baths of molten salts (mixtures of nitrates and nitrites) which make it possible to improve the corrosion resistance.
Phosphatization processes are also known which make it possible, by the creation of a surface layer of iron phosphate, to substantially improve the effects of the lubrication.
Sulfurization processes, that is to say processes for producing a layer of iron sulfide (FeS) at the surface of ferrous alloy parts with the aim of improving their properties of resistance to jamming, are also known. The parts treated by these sulfurization processes exhibit excellent resistance to rubbing, to wear and to jamming.
The invention relates more particularly to the latter type of treatment.
The sulfurization of steels and its effects on lubrication are known to a person skilled in the art and emerge, for example, from the teaching of patents FR 1 406 530, FR 2 050 754 and FR 2 823 227.
According to the teaching of these patents, the metal parts treated are immersed for 5 to 15 minutes in a bath of ionized molten salts at between 200 and 350° C. preferentially comprising potassium thiocyanate and cyanide ions, the ionization being obtained by an electrolysis, the treated part being positioned at the anode. The layer of FeS is obtained by modifying the surface layer of the ferrous alloy part. This electrolytic sulfurization in molten salts requires specific precautions in order to keep the bath in a stable condition during the passage of the current and necessitates that particular attention be paid to the recycling of the compounds used. Furthermore, this process requires a large amount of salts, which proves to be expensive.
Another solution emerges from the teaching of patent U.S. Pat. No. 6 139 973, which relates to a process which makes it possible to deposit iron sulfide by electrolysis of an aqueous solution comprising ferric ions and thiosulfate or sulfide ions at between 30 and 50° C. The treated part is this time positioned at the cathode. This process results in significant problems of adhesion of the layer of iron sulfide to the treated parts.
A sulfurization treatment by a purely chemical route, without recourse to an electrolysis, is taught in patent FR 2 860 806. The parts are immersed in an aqueous solution comprising sodium hydroxide at a concentration of between 400 and 1000 g/l, sodium thiosulfate and sodium sulfide, at a temperature of greater than 100° C. for approximately 15 minutes. The major disadvantage of this process is the natural carbonatation of the bath, which gradually renders it unusable. This inevitable deterioration imposes both economic and ecological constraints. In addition, the treatment times are lengthy, which is harmful.